In a synchronous direct-sequence code division multiple access (S-CDMA) system, users communicate simultaneously using the same frequency band via orthogonal modulation or spread spectrum. The number of orthogonal spreading codes (>1) establishes the maximum capacity of the system.
In a fixed wireless loop (FWL) S-CDMA system the chipping rate may be held constant to maintain the orthogonality of the PN codes. This implies that for higher data rates the processing gain (PG) is reduced. When the processing gain is reduced, it may become difficult or even impossible to design the short PN codes such that, for some data patterns and offsets, the adjacent cell PN codes are not highly correlated, or even perfectly correlated if the PG becomes sufficiently small.
Related to the foregoing, it is further noted that a common method to increase the data rate of a DS-CDMA system operating with a fixed chipping rate is to implement variable rate spreading codes. In this approach just a few chips modulate the each symbol in order to increase the effective symbol rate. As the spreading gain decreases, the design of the code sets having good cross-correlation properties with other code sets becomes more difficult. In cellular deployments, adjacent cells using a common frequency band must have code sets that have low cross-correlation values in order to minimize adjacent channel interference. For spreading gains on the order of 8 or 16 chips/symbol, code sets exhibiting low cross-correlation properties are very difficult to design.
This problem can be especially troublesome if the base stations, also referred to as radio base units (RBUs), are semi-synchronized, that is, if one base station maintains the same relative timing offset to other base stations. In this case if a subscriber system or unit, which may be simply referred to as a user, is operating with a PN code with high interference, the user could remain in the high interference condition and experience a much reduced signal to noise ratio (SNR) until the interferor's or the user's transmission is terminated.
Conventional orthogonal DS-CDMA systems that use a cover code, i.e., a code that is used to scramble individual codes of a code set), typically require that all CDMA channels, including control and random access channels, must use the cover code. This can cause problems, as when a long cover code is used on a random access channel, new users may require a significant amount of time to acquire the system as the phase of the cover code must be recovered. Reference with regard to the use of a cover code can be had to U.S. Pat. No. 5,751,761 by Gilhousen.
However, the use of cover codes constructed using long period linear feedback shift registers (LFSR) results in unbalanced spreading codes. For example, and referring to FIG. 6, orthogonal code sets constructed using well-known Walsh-Hadamard matrices may have P−1 balanced codes and one completely unbalanced code, typically referred to as the all one's code. In the illustrated 4×4 Walsh-Hadamard matrix there are three balanced codes (equal numbers of plus and minus codes) and the unbalanced all one's code. Without a cover code, the all one's code would be unused due to DC bias problems and large correlation with adjacent cell codes. The omission of the all one's code thus serves to reduce the capacity (i.e., the number of allowable active users) in a cell.
One technique for mitigating interference between users is disclosed in commonly assigned U.S. Pat. No. 6,023,462, Fixed Wireless Loop System that Ranks Non-Assigned PN Codes to Reduce Interference, by L. L. Nieczyporowicz, P. L. Stephenson, T. R. Giallorenzi and R. W. Steagall, the disclosure of which is incorporated by reference herein in its entirety.